Leaders from Turkey, Russia, France and Germany are meeting in Istanbul to discuss the conflict in Syria.

The United Nations has said that Damascus is blocking efforts to draft a new constitution which is key to ending the country's civil war.

Outgoing UN envoy to Syria Staffan de Mistura has been hopeful that high-level meetings will lead to a solution, but Bashar al-Assad's government has repeatedly objected to the names de Mistura included on the list and has now challenged his authority to make the list at all.